Micromachined microphones, often called “MEMS microphones” (MEMS refers to micro-electrical mechanical systems), have become an attractive alternative to conventional condenser or electret microphones. Condenser and electret microphones utilize a diaphragm that responds to sound by vibrating. The vibrations of the membrane are monitored by monitoring the capacitance of the gap between the diaphragm and a conducting plate that is close to the diaphragm. In the case of a condenser microphone, the diaphragm is made from a conductive material and is then charged using an external bias (typically in the range of 40V). For an electret microphone, the diaphragm is made from a dielectric material, usually polymer such as Teflon™, and is permanently charged during manufacture by ion implantation. The movement of the diaphragm in both condenser and diaphragm is monitored by amplifying the fluctuating voltage on the conductor that sits below the diaphragm. Since the gap between the diaphragm and conductive plate determines the capacitance of the system, the position of the diaphragm determines the electrical signal. Other methods for monitoring the movement of a diaphragm or ribbon have been developed using magnetic fields. These are known as dynamic microphones since the velocity of the moving element determines the electrical signal. Finally, still other methods for monitoring the position of a diaphragm have been developed, for example the use of a laser to monitor deflection or by frequency modulation in an AC circuit. However, the most popular microphone technology, by far, is the electret microphone.
MEMS microphones are now challenging electret microphones for market acceptance. MEMS microphones are almost exclusively built from silicon substrates using semiconductor microfabrication techniques. Since they use lithographic semiconductor processes, these microphones can be built with precise features and extremely small gaps between the diaphragm and conducting plate below the diaphragm, on the order of a few micrometers. This can increase the sensitivity of the microphone, at least in principle. In practice, MEMS microphones are not more sensitive than traditional microphones because they must use semiconductor-style materials such as silicon, silicon dioxide, or silicon nitride which are much stiffer than polymer, and because their diaphragm areas are smaller. However, MEMS microphones can be made smaller than traditional microphones and, since they are made from non-polymer materials, the MEMS microphones can handle higher temperatures than electrets. This makes them attractive for integrating in wave soldering manufacturing, where entire printed circuit boards must be exposed to heat in order to solder all components on the boards at once. Traditional electret microphones cannot survive wave soldering, and typically lose sensitivity in the process, so must be assembled onto an electronic circuit after the wave soldering step.
Some MEMS microphones build amplification and digitization electronics directly on the same substrate as the diaphragm. This reduces the size needed for the microphone and, in principle, reduces cost since a second amplifier chip is not necessary. In practice, cost is not necessarily reduced because the diaphragm manufacture does not use completely standard semiconductor processing and so the devices do not benefit from the semiconductor process to the same degree as standard CMOS electronic devices do. Furthermore, the diaphragm itself takes up considerable space on the silicon which increases cost since the cost of silicon microfabrication is almost proportional to surface area of silicon. Most MEMS microphones use separate amplifier chips which are connected to the diaphragm device during final package assembly.
All silicon MEMS microphones suffer from the need to assemble the MEMS device together with other electrical components, such as amplifier chips, passive devices and conductive leads, and further to place into a protective package. The problem with MEMS packaging is well known, and still plagues the MEMS industry. The MEMS device is typically built on a silicon wafer, but must be cut out and transferred to a metal or laminate substrate where it is electrically connected to connections on the substrate. Since the MEMS device is very fragile, this assembly is a difficult and expensive operation and is currently the cost limiting step in MEMS manufacturing. Any innovations that can simplify or eliminate this step would significantly impact the ability to deploy MEMS microphones.
The current state of art does not provide a satisfactory way to construct a micromachined microphone or micro-speaker that is truly compatible with the packaging of the acoustic device and its associated electronics. A device that can be readily constructed that is compatible with standard packaging techniques would be desirable.